In the lighting field, the high cost of energy has mandated replacement of the universally used incandescent lamp with the more energy efficient fluorescent lamp. Also, it is highly desirable to effect this lamp replacement without an undesired replacement of the fixtures already in use. Moreover, cost, efficiency, weight of the replacement, and ease of replacement of the lamps are all factors to be taken into consideration if an acceptable replacement product is to prove effective.
One known approach to the problem of replacing incandescent lamps with fluorescent lamps includes the employment of a choke-type ballast assembly. Therein, relatively rapid changes in current with time in a choke coil or the "L di/dt" chacteristic of a choke ballast is utilized to provide a pulse potential necessary to the starting of a fluroescent lamp. However, choke coils not only are costly but also undesirably add weight which may be deleterious to the fixture already in place.
Another known approach to the problem of replacing incandescent lamps with fluorescent lamps involves the employment of a resistive type ballast assembly. Therein, it is not uncommon to employ a glow starter, resistor and capacitor to provide the necessary starting and operation capabilities. However, undesired energy loss and heat generation by the resistive type assembly are characteristics deleterious to a capable replacement apparatus.
Still another arrangement for replacing incandescent lamps with fluorescent lamps includes a resistive-type ballast arrangement wherein an incandescent lamp is utilized as the resistive element. Thus, the energy previously wasted by the resistor may be utilized by the incandescent lamp to add to the output of the fluorescent lamp.
However, one of the problems associated with a resistive ballast arrangement employing an incandescent lamp as the resistive element is the improper utilization of the incandescent lamp. More specifically, the ordinary ballast arrangement includes a glow bottle shunted across the fluorescent lamp filaments and an incandescent lamp coupling one of the filaments to the voltage supply or source. The glow bottle includes the usual bimetal within a gas-containing envelope. Upon activation, the gas ionizes causing the bimetal to heat and deflect to provide a short-circuit so that current is applied to the filaments of the fluorescent lamp. Thereafter, the bimetal cools which opens the preheat circuit and the energy applied across the heated filaments is sufficient to cause ignition of the lamps.
Normally, each of the filaments of the fluorescent lamps will have a voltage drop thereacross of about ten-volts during the preheating phase which leaves a drop of about 80% of the 120-volt supply voltage across the incandescent lamp. However, during normal operation of the fluorescent lamp, the incandescent lamp will operate at about one-half or 50% of the supply voltage. Thus, a resistive ballast employing an incandescent lamp presents a serious design problem. If the incandescent lamp is designed to operate at one-half or 50% of the supply voltage, the bulb would be overstressed during the preheat period since about 80% of the supply voltage would be applied to the incandescent lamp. Thus, a short life span would result. On the other hand, an incandescent lamp designed to operate at 80% of the service voltage would receive only about 50 to 60% of the service voltage during normal operation of the fluorescent lamp. Thus, the lumen output of the incandescent lamp would fall to about 30% of the rated value which is obviously unsatisfactory insofar as efficiency is concerned.
Another problem associated with resistive ballast configurations is the difficulty of starting the fluorescent lamp. As is known, one method of inducing a starting voltage is to provide voltage spikes. However, such techniques involve added choke coils or other forms of additional apparatus which undesirably add to the cost and complexity of the apparatus. Other techniques include the utilization of an electrically floating conductive stripe within or affixed to the outer surface of the fluorescent lamp envelope. However, the known structures require a relatively high supply voltage in order to effect the capability for starting a fluorescent lamp. Since the starting voltages are limited to the 120-volt supply voltages, increased high voltage is not an alternative..